High-energy phosphorus metabolism in normal human masseter muscle will be investigated non-invasively by in-vivo phosphorus magnetic resonance spectroscopy (31P MRS). 31p MRS on human limb muscles has already contributed significantly to the understanding of muscle physiology and pathology. Before this relatively new technique can be applied to the study of pathology such as temporomandibular joint dysfunction, phosphorus spectroscopy data from healthy volunteers is needed. Therefore the aims of the proposed study are: 1) to obtain normative data on high-energy phosphorus metabolites such as adenosine triphosphate (ATP), phosphocreatine (PCr), and inorganic phosphate (Pi) as well as pH in healthy human masseter muscles at rest, and 2) to study the time course of intracellular high-energy phosphorus metabolites and pH in the healthy human masseter muscle during and after three levels of isometric voluntary contractions. Twenty non-pain, non-bruxing subjects will be examined using a whole body 2 Tesla NM/MRS system and a circular surface coil placed over the masseter muscle. Spectra will be acquired before, during, and after 50%, 75%, 100% of maximum voluntary bite force (MVBF), and at the same time bite force will be monitored. Peak integral peak integral ratios, and resonance frequency positions of individual phosphorus metabolites and their alterations with time and different levels of exercise will be determined. The results will provide information about the normal levels of intracellular ATP, PCr, Pi and pH in the healthy human masseter muscle, their time course during exercise at different bite force levels, and recovery from exercise.